Where did "Invade Greenland" come from?
I don't know.
But one way of responding to blackmail / #Kompromat is to make believable-enough threats about what will be done to the blackmailer if the kompromat is ever released / made public.
Which may have "Invade Greenland" as part of what Donald is threatening to do to Russia if Putin were to release the (presumed) kompromat.
Trump may have bought himself some wriggle-room. And would be trying to get more.